Airflow in conventional bolted steel bins and concrete silos is limited to relatively low airflow rates by vertical depth, seed characteristics, and high static pressure. Conventional vertical pressure aeration also adds “heat of compression,” typically 3° C. to 8° C. temperature rise to ambient air.
Most conventional bolted, corrugated steel bins are typically sold with the “standard” vertical pressure aeration with an airflow rate of 0.1 ( 1/10) cfm/bu. Approximately 150 hours of aeration fan time is required at 1/10 cfm/bu to completely cool stored grain with level top biological product surface. Peaked grain increases cooling time by 25-35%.
Stored grain insect populations grow exponentially. Warm to hot surface grain is ideal for insect population growth. If a bin of fresh grain has 100 fertile female adult stored product insects on July 1, by mid-September the insect population can increase to 1-3 million adult insects.
Although suction aeration cooling is a major non-chemical insect management tool, U.S. steel grain bin manufacturers will not warrantee a steel bin with a suction aeration system due to roof collapse damage potential.
Freshly harvested moist grain must have fresh air moving through it within 12-24 hours to prevent biological heating triggered by mold spore germination, leading to mold, spoilage and possible formation of toxins, which degrades market value, or makes grain unusable for livestock or poultry feed.
Much U.S. grain is dried on farms and commercial grain elevators in independent high temperature grain dryers, using high volumes of fossil fuels. Bin drying is slow, even with batch grain depths of only 4 to 5 feet, due to slow loading and unloading, and limited heat and airflow. In-bin grain drying systems burn high volumes of fossil fuel to heat drying air. In-bin deep bed drying is characterized by over-dried bottom grain and under-dried grain near the top biological product surface, such as 10% moisture bottom grain and 18% moisture surface grain for grain dried to 14-15% final moisture. Although the grain is blended some during unloading, if the top grain is not mixed well during unloading, pockets of spoilage may develop.
High temperature cross-flow grain drying—at 60-110° C. (140-230° F.)—causes multiple stress cracks of the pericarp (seed coat) of corn (maize) and damages the starchy endosperm, gluten and germ of corn and other grain. When seed or kernel temperatures exceed 40-41° C. (104.0-105.8° F), seed germination damage begins. Conventional vertical airflow for in-bin drying and aeration is limited by the high static air pressures required to move drying or cooling airflow rates through deep grain.
Much higher volumes of ambient or heated air can be forced through full bins of moist grain using cross-flow air movement technology with very low electrical power and low static pressures. Thus, in-bin cross-flow drying can provide an economical, ecological and environmentally viable method of curing moist grain, even in extremely large grain drying volumes.
Natural air and low temperature heated-air drying (air temperatures below 40° C. (104° F.) preserves grain germination at the highest quality levels. In a cross-flow bin dryer, high airflow through a deep bed of grain at 30-40° C. and 65% RH will dry 25-30% grain to about 12-13% near the vertical aerator and about 13-14% near outer wall air plenums. During unloading, this 2-3% moisture spread between kernels will mix and blend to within 0.5-1.0%. High-temperature, high-airflow cross-flow narrow (10-15 inch) column dryers typically have moisture differentials of 5-8% moisture between kernels at inner and outer perforated metal walls, which end up in storage with a wider final moisture differential between adjacent kernels.
Cross-flow aeration research in high depth-to-diameter ratio (3:1 to 6:1) silos at Oklahoma State University (Day and Nelson, 1962) in the early 1960s demonstrated that moving the same volume of air horizontally between two ducts on opposite sides of a silo required much less power and static pressure compared to the same airflow moved vertically the full height of the silo. In a 20 ft. ID×120 ft high silo, horizontal airflow moves 18-19 ft versus 120 ft vertically, a 1:6.7 diameter to height ratio. However, there is a large disparity (about 1.0:1.7 ratio) between the minimum air path directly across the middle of the silo compared to the air path following the inside silo wall (Noyes and Navarro, Editors, 2001). With the vertical center aerator discharge in all directions to wall exhaust plenums, taught in this new technology, all air paths are about the same length, so all grain receives approximately equal drying treatment.
A major problem with conventional cross-flow aeration is that silos have to be full for the cross-flow aeration to work properly. Reed (2004) describes a new patented concept of 2-duct cross-flow aeration in concrete silos. He uses a series of controls to deal with silos which are full versus not totally filled. However, as taught by Reed, the airflow and cooling zones are not uniform across the silo cross-section.
This new cross-flow aeration and drying technology involving high air-flow delivery from a central vertical aerator tube, with low static pressure, low fan power and uniform airflow, which can operate efficiently at variable bin fill levels (once grain fill exceeds 25 to 30% of the bin), taught in this patent, will be highly beneficial to worldwide grain storage systems for low cost in-bin grain drying, or for aerating grain in storage bins at high aeration rates, or at much higher drying airflow rates suitable for low cost, efficient in-bin natural air grain drying.
A new biological moisture removal phenomenon was learned during tests by Danchenko on a prototype in-bin dryer taught by this patent technology whereby heated air drying was followed by ambient air drying time of about 20-25% of heated air time (Example: 2.5 hours heated air drying at 15-20° C. temperature rise, followed by 0.5 hours ambient air drying). Danchenko repeated this heated versus non-heated air drying through multiple cycles, resulting in faster drying than with the same total length of drying time with continuous heat. This “pulsing” effect of heating grain, then cooling the grain for a short period, allowing the grain to “rest” and continue to dry using the residual grain heat between heated air cycles, resulted in an increase of 15-25% faster drying.
For safe grain storage, interstitial air equilibrium relative humidity (ERH) must be below 70% RH; 70% ERH is a critical value of water activity which defines safe upper interstitial storage air humidity limits of biological products. Microbial activity is restricted on biological products when water activity (seed, kernel or grain interstitial air humidity) remains below 70% ERH. Product temperature and moisture content are both used to determine ERH levels for biological products.
For in-bin cross-flow drying, an optional design goal is to dry continuously from relatively low fill levels (20-25%) until the bin is full to allow immediate protection of freshly harvested grain. ‘Partial-fill’ drying allows the dryer system to begin rapidly reducing grain moisture soon after wet grain reaches the drying-storage bin, quickly protecting it from mold, instead of waiting until the drying bin is full, which might mean that the partially filled bin might set for several days if harvest is interrupted by inclement weather, and the grain waiting to be dried might begin to mold. Early drying helps avoid late season storm losses.
Grain producers can dry very wet grain as soon as it can be safely harvested using this novel in-bin drying technology, because the primary power energy required is electrical energy to operate the ambient airflow system. Example: Corn is ideal for shelling between 26-28% moisture content, but can be harvested as high as 30-31% with minimal shelling damage. High moisture corn gives off surface (“free”) moisture at a very rapid rate to natural air, even with ambient air relative humidity of 70-80%, until kernel moisture drops to 23-24%. A typical fan “heat of compression” temperature rise of 3° C. (5.0° F.) will change 15° C. (59° F.), 70% RH air to 18° C. (64° F.) at 52-54% RH, suitable for drying corn to 11.5-12.0% final m.c. This drying method is ideally suited for drying food grade and ethanol fuel grains.
It may be desirable for an in-bin drying system to selectively condition grain in different vertical sections of the bin. Example: As grain moisture in the bottom of the bin is lowered, and higher moisture grain is added, airflow can be increased on the wetter grain while reducing airflow to the partially dried bottom grain, especially after lower grain drops below 70% ERH. This in-bin dryer process can be designed to allow variable airflow rates at selected vertical sectors of the dryer by control of input air through separate compartments of the vertical aerator pipe as taught in FIG. 5, or by controlling the exhaust levels of the dryer with vertically segmented air plenums, as taught in FIG. 1, or by control of multiple exhaust vent levels with a full height cylindrical plenum as taught in FIG. 2. 70% ERH moisture contents at 16° C. and 32° C. for wheat range from 13.9% to 13.0%, corn moisture ranges from 14.1% to 11.6%, and sorghum varies from 14.1% to 13.5%. At 15° C. and 35° C., soybeans moistures vary from 12.4 to 11.7% (ASAE Standards, 1993).
Valuable research, which further clarifies horizontal airflow conditions, was conducted during the early 1990s by Jayas and associates at the University of Manitoba (Jayas and Muir, 1991; Jayas and Mann, 1994). They discovered that horizontal airflow through elongated seeds and kernels, maize (corn), wheat, sunflowers, barley, rice, edible beans, etc., has 40% to 50% less airflow resistance than when the same airflow rates are moved through the same vertical distance. Thus, only 50 to 60% as much fan power is required to aerate or dry long grain or seeds with horizontal airflow compared with the same distance of vertical airflow. When aerating relatively round seeds—soybeans, sorghum (milo), millet, etc., the researchers found the airflow resistance was the same for vertical versus horizontal airflow.